Circuit-board overlaid with a copper material on both sides or in multiple layers and a method of fabricating same

ABSTRACT

A circuit-board overlaid with a copper material on both sides with a lead-tin layer on the copper material and a plate-shaped heat sink (6). To achieve desirable dissipation of heat generated by electronic components on such circuit-board, the heat sink (6) is provided with a copper surface and then a layer of silver (8) applied to it is soldered to one side of the circuit-board (1). In a method of fabricating the circuit-board, a heat sink (6) having a copper surface is provided with a silver layer (8) and is applied to the circuit board (1) with its side (7) having the silver layer (8). The heat sink (6) is pressed with the circuit board (1) and a heated platen (20) in close contact with the heat sink (6).

FIELD OF THE INVENTION

The present invention relates to a circuit-board overlaid with a coppermaterial on two sides or in multiple layers with a lead-tin layer on thecopper material and a heat sink in the form of a board.

BACKGROUND OF THE INVENTION

A conventional circuit-board is described in the publication, IPCStandard 1985, "Proposal Qualification and Performance Specification forMetal Core Boards" of the Institute for Interconnecting and PackagingElectronic Circuits, pp. 1-5. The conventional circuit-board comprisesthree parts: an internal metal core plate as the heat sink and twoplastic plates arranged on both sides of the metal core plate and bondedto it. On their sides facing away from the metal core plate, the plasticplates are copper-clad and provided with a lead-tin layer in theconventional way for mounting electronic components. The metal coreplate forms a plate-shaped heat sink by which the heat generated by theelectronic components can be properly dissipated. However, thedissipation of heat with the conventional circuit-board is not optimalbecause heat transfer from the plastic plates to the plate-shaped heatsink is prevented by the adhesive layers.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a circuit-boardoverlaid with a copper material on two sides or in multiple layers bywhich heat can be properly dissipated even from electronic components onthe circuit board that generate large amounts of heat.

To achieve the aforementioned object, the heat sink, provided with acopper surface and a silver layer applied to the latter, is solderedonto one side of the circuit-board. The method of mounting the heat sinkboard to the circuit-board leads to a desirable thermal transfer betweenthe circuit-board and the heat sink, so that the heat generated byelectronic components on the circuit-board is dissipated very well.

With the circuit-board according to the present invention, the heat sinkmay have various designs. It is advantageous if the heat sink is acopper plate because production of the circuit-board is simplified sinceonly application of the silver layer is necessary as an intermediateoperation before soldering the copper plate to the circuit-board.

According to the present invention, it is also advantageous if the heatsink is a plastic plate clad with copper and provided with a silverlayer on at least one side and having heat dissipating holes. Productionof such a plastic plate does not pose any notable problems within thecontext of fabrication of the circuit-board because the plastic platecan be manufactured with equipment available in circuit-boardfabrication. A suitable material for such a plastic plate would be aplastic having a high thermal conductivity and a coefficient of thermalexpansion approximately comparable to that of copper and silver. Such achoice of materials ensures that the bond between the plastic plate andthe circuit-board will hold reliably.

To promote the dissipation of heat when using the heat sink consistingof the plastic plate, it is advantageously plated through with copper inthe area of the heat dissipating holes. The dissipation of heat isparticularly good if the heat dissipating holes are filled with athermally conducting paste.

If the evolution of heat by the electronic components on thecircuit-board is especially high and thus a marked dissipation of heatis necessary, a metal heat discharge plate is advantageously pressedinto the plastic plate. The dissipation of heat by conduction orradiation is promoted by the heat dissipation plate.

The present invention also relates to a method of manufacturing acircuit-board overlaid with a copper material on both sides and with alead-tin layer on the copper material and a plate-shaped heat sink. Themethod provides according to the present invention that the heat sinkhaving a copper surface be provided with a silver layer. The heat sinkwith its silver-plated side is placed onto the circuit-board and theheat sink is pressed with the circuit-board, while a heated platen isapplied to the heat sink.

An advantage of the method according to the present invention is thatheating of the junction between the circuit board and the heat sink isachieved due to the heated platen so that the heat sink is soldered tothe one side of the circuit-board. Pressing here results in an intimatebonding of the heat sink to the circuit-board on the one side, so that adesirable thermal and electrical junction is ensured. The application ofheat in pressing is selected so that the lead-tin layer on the side ofthe circuit-board facing away from the heat sink does not melt.Therefore, the lead-tin layer on the side facing away from the heat sinkretains its original melting point, while the lead-tin layer between thecircuit-board and the heat sink now has a higher melting point due tosoldering and pressing. This makes it possible in soldering componentsto the side facing away from the heat sink to melt only the lead-tinlayer on this side of the circuit-board and to solder the components.The junction between the circuit-board and the heat sink is not affectedby soldering the components.

If melting of the lead-tin layer on the side facing away from the heatsink in pressing the circuit-board to the heat sink cannot be preventedby another platen at room temperature opposite the heated platen, thenaccording to another exemplary embodiment of the method according to thepresent invention, the side of the circuit-board facing away from theheat sink is cooled in pressing.

The cooling can be accomplished in different ways. It is advantageous ifanother platen opposite the heated platens is cooled during pressing.Optionally, it may be sufficient if the cooling is performed using acooling plate placed on the side of the circuit-board facing away fromthe heat sink in pressing, so that the cooling plate projects out of thepressing apparatus and thus produces the cooling effect.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an exploded side view of a circuit-board and a heat sink tobe bonded to the circuit-board.

FIG. 2 shows a side view of the circuit-board soldered to the heat sinkaccording to FIG. 1.

FIG. 3 shows a perspective diagram of a heat sink formed by a plasticplate.

FIG. 4 shows a section through a plastic plate according to FIG. 3 alongline IV--IV with a heat dissipating plate pressed into it.

FIG. 5 shows an exploded side view of a pressing apparatus for carryingout the process according to the present invention with circuit-boardand heat sink placed in it.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a circuit-board 1 having copper cladding 2 and 3 on bothsides. A lead-tin layer 4 and 5 is applied to the copper cladding 2 and3, respectively.

Furthermore, FIG. 1 shows a heat sink formed by a copper plate 6. Thecopper plate 6 is provided with a silver layer 8 on one side 7.

As explained in greater detail below, the copper plate 6 is soldered andpressed to the circuit-board 1, so that then the circuit-board 1 withthe copper plate 6 forms a cohesive laminate as shown in FIG. 2. Thelead-tin layer 5 on the circuit-board 1 causes it to be soldered to theplate 6, so that the heat generated by components (not shown) on the topside of the circuit-board 1 in FIG. 2 can be transmitted to the copperplate 6 and radiated away from the latter.

The arrangement of the circuit-board 1 and the copper plate 6 shown inFIG. 2 can then readily be provided with electronic components on side9, as is conventional in the manufacture of circuit-boards. Due to thesoldering of the circuit-board 1 to the copper plate 6, the lead-tinlayer 5 between the circuit-board 1 and the copper plate 6 has a highermelting point than the lead-tin layer 4 on side 9 of the circuit-board1.

FIG. 3 shows a heat sink formed by a plastic plate 10. The plastic plate10 is made of a material having a high thermal conductivity and acoefficient of thermal expansion corresponding approximately to that ofa copper material. Such plastic is available under code name RO 4000from Rogers, USA. The plastic is described in detail in the publication"Rogers High Frequency Circuit Materials RO 4000" of Rogers Corporation,Chandler, Ariz., USA. To impart an especially high heat dissipatingcapacity to the plastic plate 10, the plastic plate is provided withheat dissipating holes 11, which are also known as thermal vias. Theheat dissipating holes are through plated with a copper material in theinterior (not shown in FIG. 3). To further increase the heat dissipatingcapacity, the holes may be filled with a conducting paste, although thepaste is preferably applied only after the plastic plate 10 has beenbonded to a circuit-board (not shown in FIG. 3) corresponding to thecircuit-board 1 according to FIGS. 1 and 2.

To bond the plastic plate 10 to a circuit-board by soldering, it iscopper plated on its surface 12 (copper cladding 13) and then providedwith a silver layer 14--preferably by electroplating. The plastic plate10 is thus prepared to the extent of the copper plate 6 according toFIGS. 1 and 2.

As FIG. 3 shows, the plastic plate 10 is provided with a rectangularrecess 15. A heat dissipating plate 16 is pressed into the recess 15, asshown in FIG. 4. The heat dissipating plate 16 may be made of copper oraluminum.

In FIG. 5, a pressing apparatus suitable for manufacturing thecircuit-board with the method according to the present invention isshown only with a heated platen 20 and another platen 21, opposite theheated platen 20. To better illustrate the present invention, FIG. 5shows individual components that are in intimate contact between the twoplatens 20 and 21 as if they were some distance apart.

As FIG. 5 shows in detail, the circuit-board 1 according to FIGS. 1 and2 and the copper plate 6 according to the same figures is arrangedbetween the two platens 20 and 21 so that the copper plate 6 is oppositethe lead-tin layer 5 with the silver layer 8. A cooling structure or acooling plate 24 that projects outward out of the area of the platens 20and 21 is applied to side 9 of the circuit-board 1 facing away from thecopper plate 6 and is responsible for cooling the lead-tin layer 4 ofthe circuit-board 1 during pressing and soldering. Above the coolingplate 24 there is a pressure pad 25 or a pressure equalizing insert,e.g., a spring structure. When using a spring structure, cooling canalso be accomplished well using a stream of air.

In the manufacture of the circuit-board according to the presentinvention, the platens 20 and 21 are guided toward each other,compressing the parts between them with the circuit-board 1 and thecopper plate 6, with the copper plate 6 being heated by the heatedplaten 20 so that the lead-tin layer 5 melts. The circuit-board 1 isthus soldered to copper plate 6. The cooling plate 24 ensures that thelead-tin layer 4 does not melt. Thus, the lead-tin layer 4 retains itsoriginal melting point, while the melting point of the lead-tin layer 5is raised by the soldering.

After soldering the circuit-board 1 to the copper plate 6, the platens20 and 21 can be moved apart, and the laminate of the circuit-board 1and the copper plate 6 can be removed. Then the laminate can be providedwith electronic components as customary in circuit-board fabrication.

What is claimed is:
 1. A circuit-board overlaid with a copper materialon two sides of the circuit-board, the circuit-board comprising:aplate-shaped heat sink including a copper surface and a silver layer,the silver layer being soldered to one side of the circuit-board.
 2. Thecircuit-board according to claim 1, wherein the circuit-board isoverlaid with a lead-tin layer on the copper material.
 3. Thecircuit-board according to claim 1, wherein the heat sink is a copperplate.
 4. The circuit-board according to claim 1, wherein the heat sinkincludes a plastic plate and the silver layer, wherein the plastic plateincludes heat dissipating holes and wherein the heat sink is overlaidwith copper material on at least one side of the plastic plate.
 5. Thecircuit-board according to claim 4, wherein the heat dissipating holesare through plated with copper material.
 6. The circuit-board accordingto claim 4, wherein the heat dissipating holes are filled with athermally conducting paste.
 7. The circuit-board according to claim 4,wherein a metal heat dissipating plate is pressed into the plasticplate.